Systems, circuits and methods for extending the detection range of an inspection system by avoiding detector saturation
Inspection systems, circuits and methods are provided to enhance defect detection by addressing anode saturation as a limiting factor of the measurement detection range of a photomultiplier tube (PMT) detector. In accordance with one embodiment of the invention, a method for inspecting a specimen includes directing light to the specimen and detecting light scattered from the specimen. The step of detecting may include monitoring an anode current of the PMT detector, and detecting features, defects or light scattering properties of the specimen using the anode current until the anode current reaches a predetermined threshold. Thereafter, the method may use a dynode current of the PMT for detecting the features, defects or light scattering properties of the specimen.
1. A circuit for detecting light scattered from a specimen, the circuit comprising:
a photomultiplier tube (PMT) coupled for receiving and converting the light scattered from the specimen into an electrical signal, wherein the PMT comprises a cathode and a plurality of stages consisting of a multitude of dynodes and an anode, wherein the electrical signal is amplified at each successive stage to produce an output signal at the anode stage;
means coupled to the PMT for receiving the output signal along with an intermediate signal representing the amplified electrical signal at one of the dynode stages of the PMT, for supplying the output signal along to an inspection system component as long as the output signal remains below a predetermined threshold value, and for supplying the intermediate signal to the inspection system component once the output signal reaches the predetermined threshold value; and
a plurality of resistors coupled between the cathode and the anode of the PMT, wherein the plurality of resistors are separated into a first portion of serially-coupled resistors and a second portion of serially-coupled resistors, each having a separate power supply therewith, and wherein the first and second portions are not coupled in series.
2. The circuit as recited in claim 1 , wherein the predetermined threshold value comprises a maximum current level at, or just below, a current level at which the anode stage begins to saturate.
3. The circuit as recited in claim 2 , wherein the predetermined threshold value falls within a range of values encompassing about 1 uA to about 10 mA.
4. The circuit as recited in claim 2 , wherein the intermediate signal comprises the amplified electrical signal from the one dynode stage multiplied by a calibration ratio.
5. The circuit as recited in claim 4 , wherein the calibration ratio comprises the current level at the anode stage divided by the amplified electrical signal from the one dynode stage.
6. The circuit as recited in claim 1 , wherein said means comprise control logic configured for monitoring the output signal while supplying the output signal to the inspection system component and switching to the intermediate signal once the output signal reaches the predetermined threshold value.
7. The circuit as recited in claim 1 , wherein said means comprise a processor configured for executing:
a first set of program instructions for monitoring the output signal while supplying the output signal to the inspection system component; and
a second set of program instructions for switching to the intermediate signal once the output signal reaches the predetermined threshold value.
8. The circuit as recited in claim 1 , wherein the first portion of resistors are coupled to the cathode and one or more dynodes including and ending with an intermediate dynode of the PMT.
9. The circuit as recited in claim 8 , wherein the second portion of resistors are coupled to a dynode, which immediately follows the intermediate dynode, and the anode of the PMT.
10. The circuit as recited in claim 9 , wherein the intermediate signal comprises a voltage signal measured across a last resistor in the first portion of resistors.